Polymer thermal conductor: turning plastic insulator into conductor

Plastics are considered to be good insulators of heat as they restrict the movement of thermal energy. They also are excellent insulators of electricity due to their high resistivity. In any case, this protecting property is less alluring in items, for example, plastic housings for workstations and cell phones, which can overheat, to a limited extent in light of the fact that the covers trap the warmth that the gadgets deliver.

MIT scientists now have developed a polymer thermal conductor that works as a heat conductor, dissipating heat rather than insulating it. And the fascinating properties of the newly developed polymer thermal conductor includes:

Lightweight

Flexible

Conduct 10 times as much heat

Yanfei Xu, a postdoc in MIT’s Department of Mechanical Engineering said, “Traditional polymers are both electrically and thermally insulating. The discovery and development of electrically conductive polymers have led to novel electronic applications such as flexible displays and wearable biosensors. Our polymer can thermally conduct and remove heat much more efficiently. We believe polymers could be made into next-generation heat conductors for advanced thermal management applications, such as a self-cooling alternative to existing electronics casings.”

At the nanoscale, polymers are made from long chains of monomers, or molecular units, linked end to end. These chains are regularly tangled in a spaghetti-like ball. Warmth bearers experience serious difficulties traveling through this muddled chaos and have a tendency to get caught inside the polymeric growls and bunches.

Several groups have engineered polymer conductors in recent years, including Chen’s group, which in 2010 invented a method to create “ultradrawn nanofibers” from a standard sample of polyethylene. The technique stretched the messy, disordered polymers into ultrathin, ordered chains — much like untangling a string of holiday lights. Chen found that the resulting chains enabled heat to skip easily along and through the material and that the polymer conducted 300 times as much heat compared with ordinary plastics.

The insulator-turned-conductor could only dissipate heat in one direction, along with the length of each polymer chain. Heat couldn’t travel between polymer chains, due to weak Van der Waals forces — a phenomenon that essentially attracts two or more molecules close to each other. Xu wondered whether a polymer material could be made to scatter heat away, in all directions.

The current study as an attempt to engineer polymers with high thermal conductivity, by simultaneously engineering intramolecular and intermolecular forces — a method that she hoped would enable efficient heat transport along and between polymer chains.

Scientists did this by using oxidative chemical vapor deposition (oCVD), whereby two vapors are directed into a chamber and onto a substrate, where they interact and form a film. In this case, Wang flowed the oxidant into a chamber, along with a vapor of monomers — individual molecular units that, when oxidized, form into the chains known as polymers.

Yanfei Xu, a postdoc in MIT’s Department of Mechanical Engineering said, “Our reaction was able to create rigid chains of polymers, rather than the twisted, spaghetti-like strands in normal polymers.”

“We grew the polymers on silicon/glass substrates, onto which the oxidant and monomers are adsorbed and reacted, leveraging the unique self-templated growth mechanism of CVD technology.”

“Because this sample is used so ubiquitously, as in solar cells, organic field-effect transistors, and organic light-emitting diodes, if this material can be made to be thermally conductive, it can dissipate heat in all organic electronics.”

The team measured each sample’s thermal conductivity using time-domain thermal reflectance and then monitor the drop in its surface temperature by measuring the material’s reflectance as the heat spreads into the material.

On average, the polymer samples were able to conduct heat at about 2 watts per meter per kelvin — about 10 times faster than what conventional polymers can achieve. Means, the material’s properties such as its thermal conductivity, should also be nearly uniform. Following this reasoning, the team predicted that the material should conduct heat equally well in all directions, increasing its heat-dissipating potential.

“We can directly and conformally coat this material onto silicon wafers and different electronic devices,” Xu says. “If we can understand how thermal transport [works] in these disordered structures, maybe we can also push for higher thermal conductivity. Then we can help to resolve this widespread overheating problem, and provide better thermal management.”

The team includes Xiaoxue Wang, who contributed equally to the research with Xu, along with Jiawei Zhou, Bai Song, Elizabeth Lee, and Samuel Huberman; Zhang Jiang, physicist at Argonne National Laboratory; Karen Gleason, associate provost of MIT and the Alexander I. Michael Kasser Professor of Chemical Engineering; and Gang Chen, head of MIT’s Department of Mechanical Engineering and the Carl Richard Soderberg Professor of Power Engineering.